Method of depositing films on aluminum alloys and films made by the method

ABSTRACT

Method for depositing a metallic material on an aluminum alloy surface for galvanic displacement type deposition, electrodeposition or electroless deposition of a metallic film on the surface wherein the alloy surface is oxidized (e.g. anodized) to form aluminum oxide and the oxidized surface is etched to leave a partial thickness of a barier aluminum oxide on the alloy surface. The partial thickness of the barrier oxide is controlled by etching to form a porous, metallic particulate film for a thin barrier oxide, or a continuous metallic film for thicker barrier oxide. The metallic film then is electrodeposited or electroless deposited on the barrier film.

This application is a continuation-in-part of U.S. Ser. No.11/201,766filed Aug. 11, 2005, and claims benefits and priority of provisionalapplication Serial No. 60/663,659 filed Mar. 21, 2005.

FIELD OF THE INVENTION

The invention relates to galvanic displacement type deposition,electroless deposition or electrodeposition of metallic films on treatedaluminum alloys as well as to the deposited metallic films andcomponents, such as capacitor electrodes, embodying the metallic films.

BACKGROUND OF THE INVENTION

The surface of aluminum metal is spontaneously oxidized in the ambientatmosphere. This oxidation creates a dielectric film of native aluminumoxide, which has an adverse effect on electrodeposition or electrolessdeposition of metals or alloys such as Ni, Ag, Au, and Cu and theiralloys.

With respect to overcoming the problem of electrodeposition, the zincateprocess has been employed in industry for the deposition of adhesivemetallic films on aluminum. The process consists of immersing thealuminum substrate in a strong alkaline zincate solution. The nativealuminum oxide is dissolved, and zinc is deposited on the surface viagalvanic displacement of aluminum. As a result, the zinc-coated aluminumsurface becomes amenable for electrodeposition of adhesive layers ofmetals, including nickel and copper. Zincate surface activation ofaluminum has proven to be a cost-effective process for nickel bumping ofwafers prior to flip-chip assembly.

Since the zincate method is sensitive to many variables, there areincentives for developing alternative methods for the deposition ofmetals on aluminum. One alternative method has involved directelectrodeposition of copper on aluminum using several copper complexes.An electroplating procedure for nickel displacement of aluminum followedby electroless nickel deposition has also discussed. In addition, anorganic solvent has been used to lay a seed layer of copper or palladiumon aluminum substrates. Then, electroless deposition with a reducingagent was utilized to deposit substantially more copper.

In general, the galvanic displacement type deposition process proceedsvia two concurrent electrochemical reactions, which involve thereduction of ions of metals and the oxidation of the substrate surface.The driving force for this process is determined by a difference inhalf-cell potentials (e.g. redox potentials for correspondingmetal/metal ion and oxidized substrate/substrate pairs). The half-cellpotential of the reduced species has to be more positive than that ofthe oxidized substrate. Chemical etching, which effectively removes thesurface layer of oxide, precedes and/or takes place simultaneously withthe deposition of a film of metal.

The present invention involves deposition of metallic films (eitherporous or continuous) on an aluminum alloy surface to offer theopportunity for fabrication of heat dissipation systems, energyconversion and storage devices. For example, double layer capacitors areenergy storage devices that store electrical energy by sustaining anelectrical charge in a thin double layer at the interface between anionically conducting electrolyte and an electronically conductingelectrode. Potential applications for double layer capacitors includememory protection in electronic circuitry, portable electronic, andcommunication devices. Double layer capacitors can be built as either aself-standing device or a part of integrated electronic system.Mesoporous carbon, carbon nanotubes and other carbonaceous materialshave been extensively investigated for use in double layer capacitorsbecause of their very high specific surface areas. In contrast to porousmetallic films, limitations of double layer capacitors based uponcarbonaceous materials are two-fold. First, capacitance of thesematerials typically degrades at frequencies higher than 10 Hz. Thesecond limitation arises from problematic incorporation of thesecapacitors into technologically relevant materials such as silicon.

In addition, the present invention involves deposition of metallic filmson an aluminum alloy surface to offer the opportunity for fabrication ofoptical devices for surface enhanced FT-IR spectroscopy, surfaceenhanced Raman scattering and metal-enhanced fluorescence. In addition,composite materials with noble metal particles may have usefulphoto-catalytic, anti-microbial properties and tunable surface plasmonresonances. In addition, the deposition of continuous metallic films onaluminum alloys may be used for metallization of aluminum, for providinga soldering surface on aluminum and, consequently, for packaging ofelectronic devices (zincate-free nickel bumping of wafers prior toflip-chip assembly).

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method of depositinga metallic material on a substrate wherein the method includes the stepsof providing a substrate comprising an alloy of aluminum and an alloyingelement, oxidizing a surface of the alloy substrate to form aluminumoxide thereon, etching the oxidized surface to leave a partial thicknessof a barrier aluminum oxide on the alloy surface, and depositing bygalvanic displacement type deposition, electroless deposition orelectrodeposition discrete metallic nanoparticles having a particledensity of about 10⁴ to about 10¹² particles/cm² on the barrier oxide.

In an illustrative embodiment of the invention, the etching step isconducted to leave a partial thickness of the barrier oxide to increasenucleation of discrete metallic nanoparticles thereon. This embodimentof the present invention provides an aluminum alloy substrate having apartial thickness of the barrier oxide on the surface and a porous,three dimensional film structure of electrically interconnected,discrete metallic metal nanoparticles deposited by galvanic orelectroless deposition or electrodeposition on the barrier oxide. Inthis case, the nucleation density is high enough so that the neighboringmetallic particles form the electrical connections to each other. Thefilm structure includes randomly packed, generally spherical, metallicnanoparticles having a distribution of particle sizes. An electrode,such as a double layer capacitor electrode, can comprise the alloysubstrate having the barrier oxide and porous and metallic filmstructure thereon.

Still another embodiment of the present invention provides a method ofdepositing a metallic material on a substrate wherein the methodincludes the steps of providing a substrate comprising an alloy ofaluminum and an alloying element, oxidizing a surface of the substrateto form aluminum oxide thereon, etching the oxidized surface to leave apartial thickness of the barrier oxide on the alloy surface, anddepositing by electroless deposition or electrodeposition a continuousmetallic film on the barrier oxide. As an illustration, the metallicfilm is composed of Ni. In an illustrative embodiment, the etching stepis conducted to almost completely remove the barrier oxide in order todeposit a continuous metallic film.

Features and advantages of the invention will become more readilyapparent from the following detailed description taken with thefollowing drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a SEM (scanning electron micrograph) collected after theelectroless deposition of silver particles on anodized and chemicallyetched aluminum-copper alloy film for 24 hours. FIG. 1B is a highmagnification SEM of the deposit cross-section.

FIG. 2A are cyclic voltammograms obtained for silverparticles/aluminum-copper alloy film electrode, and FIG. 2B showscharging current dependence upon the scan rate. While the solid lineshows the fit over the whole range of scan rates, two dotted lines showthe fit over two regions of slow and fast scan rates.

FIG. 3A is a Bode plot (data as symbols, modeling as lines) of EISresults, FIG. 3B is a Nyquist plot. FIG. 3C shows the equivalentcircuit.

FIG. 4 shows polarization curves of the anodized and etched Al-Cu alloyfilm electrode where curve (a) is before and curve (b) is after additionof AgNO₃ to the final concentration of 3.0 mM.

FIG. 5 is a SEM micrograph collected after galvanic displacement typedeposition of silver for 40 min.

FIG. 6 is a SEM micrograph collected after electrodeposition of silverat—1.3 V for 40 min.

FIG. 7 is a Bode plot of EIS data collected at OCP after galvanicdisplacement type deposition of silver by galvanic displacement for 40min. Experimental data are symbols and results of modeling are solidlines. FIG. 7A shows the equivalent circuit is shown as an inset.

FIG. 8 is a Bode plot of EIS data collected at OCP afterelectrodeposition of silver at—1.3 V for 40 min. Experimental data aresymbols and results of modeling are solid lines.

FIG. 9 shown the equivalent circuit used for modeling of theelectrodeposited porous film of silver (FIG. 8).

FIGS. 10A-10F are diagrams of electrodeposition on patterned Al-Cu alloyfilm substrates where FIG. 10A shows photolithographic patterning; FIG.10B shows anodization at 90 V for 2-3 min to form a dense layer ofAl₂O₃; FIG. 10C shows photoresist removal to expose the underlying Al-Cualloy layer or film; FIG. 10D shows anodization at 50 V for 20 min toform porous Al₂O₃; FIG. 10E shows chemical etching of porous A1₂0₃; andFIG. 10F shows electrodeposition of silver.

FIG. 11A is a SEM micrograph of patterned features, showing fourcircular regions of electrodeposited silver and FIG. 11B is a SEMmicrograph of a patterned feature with electrodeposited silver.

DETAILED DESCRIPTION THE INVENTON

The invention provides a method for the galvanic displacement typedeposition, electroless deposition or electrodeposition of a metallicfilm on a treated surface of an alloy of aluminum and an alloyingelement. The aluminum alloy can comprise an alloy of aluminum and one ormore alloying elements to provide a binary, ternary, quaternary, etc.aluminum alloy. For purposes illustration and not limitation, thealloying element can include, but is not limited to, one or more of Au,Cu, Cr, Mn, Mo, Ni, Si, Ta, Ti, or Zn, or combinations thereof.

The invention can be practiced to deposit a variety of metallic layersor films on the aluminum alloy surface. For purposes of illustrating andnot limiting the invention, the invention is useful to deposit ametallic layer or film comprising Au, Ag, Pd, Cu, Ni, Pb, Cr, Fe, W, Mo,or Co wherein the term metallic film includes a layer or film comprisinga metal or an alloy of these metals one with another or with anotherdifferent metal, or mixture of two or more metals, depositedconcurrently or sequentially to provide a metallic film on the surfacewhereby the deposited metallic film comprises a binary alloy deposit,ternary alloy deposit, quaternary alloy deposit and so on. The metallicfilm can have a thickness in the range of 1 nm to 10 microns forpurposes of illustration and not limitation; however, practice of theinvention is not limited to any particular thickness of the metallicfilm since any suitable metallic film thickness can be deposited.

The method envisions providing an aluminum alloy surface that is treatedin a manner effective to render the alloy surface amenable to galvanicdisplacement type deposition, electrodeposition or electrolessdeposition of a metallic film thereon. Galvanic deposition andelectroless deposition both can occur with no external electrical powerrequirement such that galvanic deposition is considered by some to be aform of electroless deposition. Galvanic deposition generally is adeposition process in which the substrate comprises a less noble elementthat acts as a reducing agent for a metal cation dissolved in thedeposition solution to effect deposition of the metal. Electrolessdeposition involves providing a reducing agent in the depositionsolution containing the metal cation to be deposited to effect itsdeposition on the substrate, which may not be less noble than the metalto be deposited. The alloy surface to be treated pursuant to theinvention can include, but is not limited to, any type of aluminum alloysubstrate, layer, film, or other surface on which the metallic filmcomprising a metal or alloy is to be deposited by electrodeposition orelectroless deposition.

The method of the invention involves treating the aluminum alloy surfaceby oxidizing the surface to form aluminum oxide thereon wherein thealuminum oxide comprises an outer porous aluminum oxide and an innerbarrier aluminum oxide adjacent the alloy surface. The anodizationresults in enrichment of the alloying element present in the aluminumalloy, such as for example Cu, under and/or in the barrier aluminumoxide.

The invention can be practiced using anodizing to oxidize the surface toform aluminum oxide thereon. However, practice of the invention is notlimited to any particular anodizing process. For example, the anodizingprocess can vary with particular type of surface to be treated. Anyconventional anodizing process can be used with the type of electrolyteand parameters of anodizing, such as anodization voltage, electricalcurrent density, temperature and electrolyte acidity being selected asdesired. For example, the anodizing process can be conducted in anyconventional aqueous electrolyte that includes, but is not limited to,solutions of oxalic acid, sulfuric acid, phosphoric acid, chromic acid,and mixtures of two or more of these acids. The invention also can bepracticed using other oxidizing processes to form aluminum oxide on thesurface. For purposes of illustration and not limitation, alternativeoxidizing treatments to anodization include polishing, alkaline etching,acid pickling, electropolishing, heating up to 700° C. in an oxygenbearing atmosphere such as air, and any other treatment, which resultsin oxidation of the aluminum alloy surface and formation of aluminumoxide on the surface.

The aluminum oxide then is etched in a manner to remove the porous outeraluminum oxide and partially remove the barrier aluminum oxide to leavea portion of its original thickness on the alloy surface. The etchingstep is conducted for a time in a selected etchant to leave a controlledpartial thickness of the barrier aluminum oxide remaining on the alloysurface in dependence upon the type of metallic film to be subsequentlydeposited.

For example, in an illustrative embodiment of the invention, the etchingstep is conducted to leave a partial thickness of the barrier aluminumoxide that is effective to enhance nucleation of discrete metallicnanoparticles on the barrier oxide during subsequent electroless orelectrodeposition so as to form a porous, metallic particulate film. Forpurposes of illustration and not limitation, in this embodiment, thethickness of the partial thickness of the barrier oxide remaining on thealloy surface can be about 50 nm or less depending on depositionparameters, such as overpotential for electrodeposition, and presence ofadditives in the solution.

The resulting porous, metallic particulate film deposited on the Alalloy substrate comprises the partial thickness of the barrier oxide onthe alloy surface and a porous, three dimensional film structure ofelectrically interconnected metallic metal nanoparticles deposited bygalvanic or electroless deposition or electrodeposition on the barrieroxide. The film structure includes randomly packed, generally sphericalmetallic nanoparticles having a distribution of particle sizes and highparticle density in the range of about 104 to about 10¹² particles/cm².For example, the metallic nanoparticles can have a particle diameter inthe range of about 20 nm to about 1000 nm. An electrode, such as adouble layer capacitor electrode, can embody such alloy substrate havingthe partial thickness of the barrier oxide and this film structurethereon.

In another illustrative embodiment of the invention, the etching step isconducted to leave a partial thickness of the barrier aluminum oxidethat is thin enough to yield a continuous (non-porous) metallic film onthe barrier oxide during subsequent electroless or electrodeposition.For purposes of illustration and not limitation, in this embodiment, thethickness of the partial thickness of the barrier oxide remaining on thealloy surface can be about 5 nm or less depending on depositionparameters, such as overpotential for electrodeposition, and presence ofadditives in the solution.

Practice of the invention is not limited to any particular etchingprocess. For example, the etching process can vary with the particulartype of aluminum alloy surface to be treated. Any conventional etchingprocess can be used with the type of etchant and time of etching beingselected empirically to achieve a desired etched barrier aluminum oxideof controlled partial thickness described above depending upon themetallic film to be deposited. For example, the etching process can beconducted in any conventional acid etchant that includes, but is notlimited to, an acidic aqueous solution (phosphoric acid, oxalic acid,sulfuric acid) or a mixture of an acid and an inhibitor of aluminumoxidation, such as chromic acid. Other inhibitors can be used as analternative to chromic acid. Etching also can be performed in analkaline solution of sodium hydroxide, or any other hydroxide.

Although the Examples set forth below involve anodizing using an aqueousoxalic acid solution and certain anodizing parameters and acid etchingusing an aqueous solution of phosphoric acid and chromic acid, these areoffered merely for purposes of illustrating and not limiting theinvention. Similarly, although the Examples are described with respectto a surface of a thin film or layer of an alloy of Al and Cu where Aland Cu are present in respective amounts of 99.5 weight % and 0.5 weight% of the alloy, the Examples are offered merely for purposes ofillustrating and not limiting the invention.

Example 1 describes a method pursuant to an illustrative embodiment ofthe invention wherein an aluminum-copper alloy is anodized and thenchemically etched followed by electroless deposition of silver on thetreated alloy surface.

EXAMPLE 1 Galvanic Displacement Type Deposition

In particular, aluminum-copper alloy film covered wafers used in thisExample were fabricated as follows: First, a 600-nm layer of SiO₂ wasthermally grown by steam oxidation of each silicon wafer. Second, a 3 -μm thick layer Al-Cu alloy (99.5 weight % aluminum and 0.5 weight %copper) was deposited on the layer of Sio₂ by physical vapor deposition(PVD). Third, each wafer having the Al-Cu alloy layer was anodized in anelectrochemical cell at 50 V dc for 20 min in 3% by weight oxalic acidaqueous solution at 0° C. The electrical contact was made to the topmetallic layer outside the electrochemical cell. The steady statecurrent density, established after 5 minutes of anodization, wasapproximately 1.4 mA/cm . Preliminary experiments revealed that theentire 3 μ m thick metallic layer was anodized in approximately 80-85min. Thus, anodization for 20 min consumed about 0.75 μ m of themetallic layer. Following anodization, the porous and barrier layers ofaluminum oxide were etched in a mixture of 0.4 M H₃PO₄ and 0.2 M H₂CrO₄acids at 60° C. and for approximately 1 hour to remove the outer porousaluminum oxide and partially remove the thickness of the barrieraluminum oxide. Chromic acid is known to be an inhibitor for corrosionof aluminum and was used to decelerate the dissolution of the remainingmetallic layer. Galvanic displacement of Al-Cu by silver (1.5 mM AgNO₃)was performed in a mixture of 0.4 M H₃PO₄ and 0.2 M H₂CrO₄ acids, at 60°C. with no stirring and for approximately 48 hours. Anodization of Al-Cualloy films was carried out with a Pt mesh counter electrode. Allelectrochemical measurements were carried out using a three-electrodecell with the Pt mesh counter electrode and a reference electrode(either a Pt wire or a Ag/AgCl electrode). Cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS) were performed with anIM6-e impedance measurement unit (Zahner), in the same mixture of 0.4 MH₃PO₄ and 0.2 M H₂CrO₄ acids, at 22° C. EIS data were acquired at opencircuit potential (OCP) over a frequency range between 0.1 Hz and 100kHz and with a potential amplitude of 5 mV. The CV and EIS data werenormalized to the geometric electrode area (1.4 cm 2). During the courseof electroless deposition, the depletion of silver and accumulation ofcopper in the deposition solution were monitored with an inductivelycoupled plasma (ICP) atomic absorption Perkin-Elmer spectrophotometer.The surface morphology of deposited silver particles was evaluated by aHitachi (S-5200) scanning electron microscope operated at 2-3 kV.

Galvanic displacement proceeds via two concurrent electrochemicalreactions: the reduction of metal ions and oxidation of the substratesurface. If a substrate has a layer of surface oxide, electrolessdeposition follows and/or coincides with chemical etching of the oxidelayer. Little is known about the displacement deposition of silver onaluminum films containing copper in acidic media. Therefore, it isimportant to address the issue of the substrate pretreatment for theelectroless deposition of silver on aluminum/copper films.

Three main observations can be made from work related to this Example.First, under the conditions of this Example, no electroless depositionof silver was observed with the pure aluminum substrate (99.997 % purealuminum foil), which was processed in the same way as alloy filmscontaining 99.5 wt % Al and 0.5 wt % Cu. Second, electroless depositionof silver did not take place on either of these two substrates ifanodization and etching steps were omitted. Third, the Al-Cu alloy filmsare made amenable for electroless deposition of silver by anodizationfollowed by complete chemical etching of porous aluminum oxide andpartial chemical etching of barrier aluminum oxide. Upon completion ofchemical etching, the thickness of barrier aluminum oxide remaining onthe treated alloy surface is approximately 1.5 nm. A combination ofanodization and chemical etching results in the copper enrichment in andunderneath the thin layer of barrier aluminum oxide. This enrichmentenables the charge transfer between silver cations and metallicsubstrate. In contrast to pure aluminum substrates, the reduction ofsilver cations on anodized and etched Al-Cu alloy substrates becomespossible. Thus, silver particles are deposited by galvanic displacement.

With respect to the species that are oxidized during the electrolessdeposition, after approximately 48 hours of electroless deposition, thedepletion of silver in the electrochemical cell corresponded to thedeposition of 4.0 mg (37 μ mole) of silver. In contrast, only 0.45 mg (7μ mole) of copper was determined to accumulate in the depositionsolution. If copper were the only reducing species, the molar ratiobetween amounts of depleted silver and accumulated copper would be 2to 1. The high molar ratio of approximately 5 suggests that both copperand aluminum are oxidized during the electroless deposition. One canexpect that aluminum is oxidized because aluminum alloys containingcopper are known to have lower corrosion resistance and are moresusceptible for an attack by an oxidizing agent (silver cations in ourcase) than pure aluminum.

Additional evidence that aluminum along with copper is oxidized duringthe electroless deposition of silver comes from examining the steadystate mixed potential, E_(mp), established during the electrolessdeposition of silver. Based upon the mixed potential theory, E_(mp) islocated between the formal potentials of the reduced and oxidizedspecies. Under our experimental conditions, E_(mp) (−0.7/−0.5 V vs.Ag/AgCl) was more negative than the formal potential of Cu²⁺/Cu. Such anegative value can be rationalized if the partial reaction of oxidationinvolves a species with a sufficiently negative formal potential (inthis case aluminum).

FIG. 1A shows that galvanic displacement deposition for 48 hoursproceeds via the formation of a network of spherical and randomly packedparticles of silver with a broad distribution of particle diameters. Theparticle diameter varies from 50 to 600 nm and the mean particlediameter is about 200 nm. Particles overlap with the neighboringparticles and are electrically interconnected to each other. Theinterconnection of particles enables the charge transfer between thealuminum-copper alloy substrate and silver cations in the solution. As aresult, the growth of particles does not stop upon the deposition of thefirst layer of silver particles adjacent to the aluminum-copper alloysurface. Rather, galvanic displacement deposition proceeds with theformation of a 1-2 μ m thick multi-layer structure, where the averageparticle diameter (the thickness of a single layer) is smaller than theoverall thickness of the porous layer of silver particles. FIG. 1A showsthat the particle density in a single layer is about 10⁹ particles cm⁻².The inter-particle space allows for the electrolyte access among silverparticles. Thus, by analyzing FIG. 1A, one can conclude that themulti-layer and porous structure composed of interconnected silverparticles is expected to have a high ratio between electrolyteaccessible and geometric surface areas.

Careful examination of individual silver particles (FIG. 1B) revealsthat silver particles are slightly rough and interconnected to eachother by other particles. Shown as white spots on the surface of 200 nmparticles are silver nano-particles, which tend to grow into 200 nmparticles over the time of galvanic displacement deposition. Analysis ofFIG. 1A, 1B confirms that nucleation of new silver particles is aprogressive process, which coincides in time with growth of existingsilver particles.

In order to characterize the dielectric properties of the silverparticles/electrolyte interface, a series of cyclic voltammograms wascollected at scan rates of 25, 50, 100 and 200 mV/sec (FIG. 2A). Whilethe negative potential limit is determined by water electrolysis, thepositive potential limit is defined by copper oxidation. Appliedpotentials (FIG. 2A) are more negative than potentials, which result insignificant oxidation of silver. FIG. 2A demonstrates that the currentmagnitude does not significantly change during either cathodic or anodicscan in the potential window of about 0.6 V between −0.5 and 0.1 V vs.Ag/AgCl. Thus, at the reported range of scan rates the faradaic currentdue to any possible oxygen and/or proton reduction is small. Moreover,the observed current is linearly proportional to the scan rate (FIG.2B), which indicates its capacitive origin. This capacitance,(C_(area)), normalized to the electrode geometric area and determinedover the shown range of scan rates, reaches a value of 1.7±0.2mF/cm^(2.) We note that the capacitance is larger at slow scan rates (25and 50 mV/s) than at fast scan rates (100 and 200 mV/s) as indicated bythe slopes of dotted lines (FIG. 2B). This observation is consistentwith previously reported observations that the double-layer capacitanceof a porous electrode depends upon the time scale of measurements.Measurements over a long time scale result in the deep penetration ofpotential perturbation into the porous structure and a large sampledsurface area. Thus, it is not surprising that the linear regressionanalysis of cyclic voltammograms with slow scan rates (25 and 50 mV/s)produces larger values of the double layer capacitance (2.0 mF/cm²) thanthat obtained by analyzing the whole range of scan rates.

Electroless deposition of silver and nickel has been previously shown toincrease the double layer capacitance. A high capacitance of themetal/electrolyte interface has been explained by an increased surfaceroughness (a ratio between real and geometric surface areas). However,the capacitances reported in literature for electroless deposition ofmetal particles have been only one order of magnitude higher than onestypical for the metal / electrolyte interface (20-40 μ F/cm²). Incontrast, C_(area) of the electroless deposited silver particlespursuant to this Example of the invention exceeds these values almost bytwo orders of magnitude.

It would be worthwhile to estimate the electrolyte accessible surfacearea of silver particles per gram of silver (S_(a), m²/g) and comparethis value with the specific surface area of spherical particles ofsilver per gram of silver (S_(g), m²/g). Equation (1) connects S_(a)with the gravimetric capacitance of silver particles/electrolyteinterface per gram of deposited silver (C_(mass); F/g) and specificcapacitance of smooth silver/electrolyte interface (C_(spec)=20×10⁶F/cm²).S _(a)=C_(mass)/C_(spec)=(C_(area)×A)/(m×C_(spec))=3.5m²/g   (1)

In Equation (1), A is the geometric electrode area (1.4 cm²) and m isthe mass of deposited silver (4.0 mg). Thus, one can calculate thatC_(mass) is equal to 0.70 F/g and S_(a) is equal to 3.5×10⁴ cm^(2/)g(3.5 m²/g). For spheres of silver, S_(g) can be calculated according toEq. (2), where ρis silver density (11×10⁶ g/m³) and D is the meandiameter of spheres (200 nm), as determined from FIGS. 1A, 1B.S _(g)×6/(ρ×D)×2.7 m²/g   (2)Comparison of these two values indicates that S_(a) is larger than S_(g)most likely due to slight roughness of silver particles (FIG. 1B). Thus,the surface area of silver particles is completely (exceptinterconnected areas) utilized to increase the double layer capacitance.

S_(g) of interconnected silver particles is lower than S_(g) reportedfor activated carbonaceous materials (˜1000 m2/g). The differencebetween these specific surface areas is compensated for by one order ofmagnitude, when one considers that the atomic weights of carbon andsilver are, respectively, 12 and 109 g/mol. Moreover, S_(g) ofcarbonaceous materials is usually determined by gas adsorption methods.Thus, the electrolyte accessible area, which is available for chargedspecies, may be appreciably smaller than the specific surface area dueto possible hydrophobicity. As shown in the previous paragraph, theopposite trend is observed for interconnected silver particles. Toalleviate the difference in S_(g) for two materials, S_(g) of silverparticles/electrolyte interface may be increased with the deposition ofsmaller particles, which have a higher inner surface to volume ratiothan one obtainable with 200 nm particles.

In addition to the electrolyte accessible area and gravimetriccapacitance, another point to evaluate technological utility ofelectroless deposition of silver particles for the fabrication of doublelayer capacitors is the frequency response. This variable is obtainedusing EIS, electrochemical impedance spectroscopy. Although EIS resultsare frequently presented in literature dealing with super-capacitors,the experimental data are rarely analyzed with an equivalent circuit.The difficulty in modeling of porous electrodes results from thedistributed nature of the double layer capacitance along the pore lengthin the direction perpendicular to the electrode surface. The chargetransfer resistance and Warburg impedance have to be considered in thepresence of a faradaic reaction. Due to the distributed character of theinterfacial impedance, the impedance of the porous electrode (includingthe multi-layer and porous structure composed of interconnected silverparticles) is properly described by the transmission line modeldescribed in R. de Levie, Electrochem Acta 8, page 751 (1963). The modelcan be applied to either straight or tortuous pores.

The EIS results obtained for the silver particles/aluminum-copperelectrode are summarized in FIG. 3A, 3B. While FIG. 3A shows the Bodeplot (both the magnitude and phase), FIG. 3B shows the Nyquist plot. Theequivalent circuit used for modeling of EIS data is shown in FIG. 3C.According to the previously developed transmission line model(www.zehner.de Application Note 01 (1997), the impedance of the porouslayer consists of three elements: R_(pore), the ionic resistance ofpores filled with the electrolyte, R_(silver,) the electronic resistanceof the solid layer (interconnected silver particles) and Z_(q), theimpedance of the interior interface between silver and electrolyte.Z_(q) is modeled as a serial connection of two constant phaseelement/resistor combinations. The constant phase element (CPE) is oftenused instead of a pure capacitance to describe interfacial dielectricproperties. EIS were performed at OCP while the electroless depositionof silver was taking place. Therefore, resistors (R₁ and R₂) model thecharge transfer across the silver particles/electrolyte interface due toreduction of silver cations and concurrent oxidation of thealuminum/copper substrate. It is worthwhile to note that the EIS databetween 0.1 and 100 Hz can be adequately modeled with R_(pore),R_(silver) and Z_(q) containing only a single (CPE₁ R₁) combination.However, satisfactory description of the frequency response between 100Hz and 100 kHz requires that the second (CPE₂ R₂) combination beintroduced. The exact origin of the second (CPE₂ R₂) combination isundefined. In addition to the impedance of the porous layer, R_(s) isused in the equivalent circuit to model the bulk electrolyte resistance.The underlying silicon substrate does not appear in the analysis of EISdata because the electrical connection was made to the top metalliclayer.

The magnitude of CPE₁, which describes the silver particles/electrolytecapacitance, is determined to be 1.4±0.1 mF×s^(α−1)/cm², a value that isslightly smaller than one obtained by CV (assuming that α₁≃1). Thesmaller magnitude of CPE₁, results from the fact that EIS is performedover a shorter time scale than CV. Other parameters of the equivalentcircuit, which is used to fit the EIS data to the transmission linemodel, are calculated as follows: R_(s) =27±1Ω×cm², R_(pore) =370±60Ω×cm², R_(silver) =31±2Ω=cm ², α₁=0.98±0.01, R₁=3.9±1.6 kΩ×cm², CPE₂32200±25 μ F×s^(α−)/cm², α₂=0.87±0.02, R₂=1.9±0.2 Ω×cm². These valuesagree with the qualitative analysis of EIS data. For example, the totalcell impedance at frequencies higher than 10 kHz is indicative of a pureresistor of 58 Ω×cm² (FIG. 3B). Due to negligible impedance ofcapacitors, this value is a sum of the bulk electrolyte resistance,R_(s), and the electronic resistance of interconnected silver particles,R_(silver). At frequencies between 200 Hz and 10 kHz, a combination ofthe ionic resistance of pores, R_(pore), and increased impedance of CPE₁produces a depressed semi-circle (FIG. 3B). At frequencies between 10and 200 Hz, the impedance of the interior interface between silverparticles and electrolyte, Z_(q), becomes dominant, which results in astraight line on the Nyquist plot (FIG. 3B).

The Nyquist plot for porous electrodes is typically divided into tworegions by the “knee” frequency. As discussed in the previous paragraph,the impedance in the high frequency region (≧200 Hz) is due the porouselectrode structure, the impedance in the low frequency region (≦200 Hz)is dominated by the whole interior interface between silver particlesand electrolyte. Examination of FIG. 3A reveals that the “knee”frequency is located at about 200 Hz. This value suggests that theelectrical energy can be stored in the double-layer capacitor atfrequencies up to 200 Hz. In contrast, the majority of carbon-basedsuper-capacitors, with a few exceptions, show the “knee” frequencyaround a few Hz. Therefore, their capacitive behavior deterioratesbetween 10 and 100 Hz. The superior performance (the high “knee”frequency) achievable with the capacitor described here is believed toresult from the suitable porosity (the inter-particle distance of about20-40 nm) and easy access of the electrolyte among the deposited silverparticles. The high “knee” frequency of 200 Hz is an important advantageof the capacitors reported in this paper.

A critical issue in the design of high power density super-capacitors isthe low electronic resistivity of the porous electrode structure. Forexample, it ahs been reported that the contact resistance between theelements (particles or fibers) in the electrode must be very small.Among metals, silver has the lowest electrical resistivity (1.6 μΩ× cm).However, the electronic resistance of interconnected silver particles,R_(silver,) is comparatively high. This value is determined by tinyinterconnected areas among silver particles as well as the contactresistance of silver particles to the aluminum substrate. One canspeculate that the interconnected areas can be increased and,accordingly, R_(silver) can be decreased with the deposition of smallerparticles. Future study will aim at the deposition of 30-60 nm particlesof silver, which are expected to have a higher inner surface to volumeratio and larger interconnected areas than those reported in this paper.

To summarize this Example, the utilization of electroless deposition ofsilver by the galvanic displacement mechanism on aluminum-copper alloyfilms has been demonstrated in order to fabricate a multi-layer andporous network composed of electrically interconnected silver particles.This structure has a high ratio between electrolyte accessible andgeometric surface areas (≃100). Two electrochemical techniques (CV andEIS) independently suggest that the capacitance of the silverparticles/electrolyte interface normalized to the electrode geometricarea (1.7 mF/cm²) exceeds those typical for the smoothsilver/electrolyte interface (20 μF/cm²) by two orders of magnitude.Evaluation of electrochemical data and SEM micrographs suggests that thesurface area of silver particles is completely accessible to theelectrolyte (except interconnected areas). The gravimetric capacitanceof silver particles/electrolyte interface per gram of silver is 0.70 F/gand the useful DC potential range is approximately 0.6 V. Analysis ofthe Nyquist plot shows that the network of silver particles is capableof storing electrical energy at frequencies up to 200 Hz. This value ishigher than those typically reported for carbon based double layercapacitors. In addition, use of silicon wafers with aluminum/copperalloy films is attractive because these wafers are frequently employedin standard micro-fabrication lines. Given these two advantages, thedescribed capacitor could find applications for special electroniccircuits where a high frequency response is required.

Example 2 describes a method pursuant to another illustrative embodimentof the invention wherein an aluminum-copper alloy surface is anodizedand then chemically etched followed by galvanic deposition orelectrodepostion of silver on the treated alloy surface.

EXAMPLE 2

In particular, aluminum-copper alloy covered wafers used in this Examplewere fabricated as follows: First, silicon wafers with a 600 nm thicklayer of SiO₂ overlayed with a 3 μ m thick layer of 99.5 wt % Al and 0.5wt % Cu (deposited by physical vapor deposition) were used in allexperiments. The Al-Cu alloy layer was anodized with a Pt mesh counterelectrode at 50 V DC for 20 min in 3 wt % H₂C₂O₄ acid and at 0° C. Theelectrical contact was made to the top metallic layer outside theelectrochemical cell. After anodization, the porous and barrier layersof aluminum oxide were etched in a mixture of 0.4 M H₃PO₄ and 0.2 MH₂CrO₄ acids at 50 ° C. for approximately 90 minutes to remove the outerporous aluminum oxide and partially remove the thickness of the barrieraluminum oxide. Upon completion of etching the specific capacitance ofbarrier aluminum oxide was determined to be 5.8 μ F/cm². Assuming thatthe dielectric constant was 8.6, the layer of barrier aluminum oxideremaining on the treated alloy surface was estimated to be 1.3 nm thick.H₂CrO₄ was used to inhibit the dissolution of the remaining Al-Cu alloyfilm. Following anodization and chemical etching, either galvanicdisplacement type or potentiostatic electrodeposition of silver (3.0 mMAgNO₃) was performed in a mixture of 0.4 M H₃PO₄ and 0.2 M H₂CrO₄ acidsat 50° C., with no stirring and for 40 min. Potentiodynamic polarizationmeasurements and electrochemical impedance spectroscopy (EIS) wereperformed with an IM6-e impedance measurement unit (Zahner). Allelectrochemical measurements were carried out using a three-electrodecell with a Pt mesh counter electrode and a reference electrode (eithera Pt wire or a Hg/HgSO₄ electrode) in the mixture of 0.4 M H₃PO₄ and 0.2M H₂CrO₄ acids at 50 ° C. For polarization measurements, the potentialstep was 2 mV and the time delay to sample the steady state current was1 s. EIS data were acquired at open circuit potential (OCP) over afrequency range between 0.5 Hz and 100 kHz, with a potential amplitudeof 5 mV and were normalized to the geometric electrode area (1.4 cm²).The surface morphology of deposited silver particles was evaluated by aHitachi (S-5200) scanning electron microscope operated at 2-3 kV.

According to the mixed-potential theory for galvanic displacement typedeposition, the steady state mixed potential, E_(mp), is determined bythe partial currents of reduction and oxidation reactions, which areequal to each other in the magnitude and opposite in the sign. When therate of partial cathodic reaction is increased by adding a suitablereducible species, E_(mp) shifts in the anodic direction. In thisExample, the effect of addition of AgNO₃ on both the exchange currentdensity and E_(mp) is further investigated with potentiodynamicpolarization experiments (FIG. 4) performed prior to either galvanicdisplacement type or electrodeposition. Three features are notable bycomparing the representative E-log (j) curves of stationary Al-Cu alloyelectrodes, (a) after anodization and etching and (b) upon addition ofAgNO₃. First, before addition of AgNO₃, OCP of −0.88 V is determined bypartial reactions involving oxidation of the Al-Cu substrate andreduction of protons and/or residual oxygen. Upon addition of AgNO₃,E_(mp) shifts to ≃230 mV more positive indicating that reduction ofsilver cations becomes the dominant cathodic reaction. Second, theinduced anodic shift of E_(mp) results in a larger overpotential for theoxidation reaction, which increases the partial anodic current densityand accelerates oxidation and dissolution of the Al-Cu substrate. Third,as a result of higher rates of cathodic and anodic reactions, theexchange current density increases from (a) 2×10⁻⁷A/cm2 to (b)7×10⁻⁶A/cm². FIG. 4 illustrates that both the exchange current densityand E_(mp) are correlated with the rates of partial electrode reactions.Therefore, polarization experiments are informative to predict whethergalvanic displacement type deposition proceeds with a reasonabledeposition rate.

FIG. 5 shows the state of the surface of anodized and etched Al-Cu alloysubstrate after deposition of silver by galvanic displacement for 40min. The black scallops with white edges can be observed behind silverparticles. These shallow scallops are formed on the surface as a resultof anodization and subsequent chemical etching. The coverage of randomlydistributed particles of silver is about 4×10⁹ particles cm⁻². Arelatively large D_(mean) of 180±110 nm and S_(g) of 3 m²/g indicatethat new particles do not nucleate as fast as existing particles grow.

FIG. 4 allows one to determine how the cathodic electrode polarizationincreases the rate of silver reduction. While the exchange currentdensity at E_(mp) (−0.65 V) is 7×10⁻⁶ A/cm², the cathodic currentdensity at −1.3 V (0.65 V more negative than E_(mp)) is 6×10⁻⁴ A/cm ².One can hypothesize that the cathodic electrode polarization wouldincrease the rate of nucleation of new particles rather than the growthof existing particles. In order to investigate this hypothesis, theAl-Cu alloy electrode was polarized to −1.3 V vs. Hg/HgSO₄ for a timeinterval of 40 min, during which the cathodic current density graduallyincreased from 0.6 to 2 mA/cm². The faradaic efficiency for silverreduction at −1.3 V is about 40% due to reduction of protons and/orresidual oxygen. This assumption is based upon the comparison of curves(a) and (b) (FIG. 4) and observing an inflexion point in the curve (b)around −1.1 V. The inflexion point indicates that the faradaic currentdue to residual faradaic reactions starts to exceed the faradaic currentdue the electrodeposition of silver. Comparison of curves (a) and (b)suggests that the current density due to residual faradaic reactions ishigher in the latter case. This observation can be explained by a highcatalytic activity and a high surface area of silver particleselectrodeposited during a potentiodynamic scan in comparison with thoseof the anodized and etched Al-Cu film. Taking into account the faradaicefficiency for silver reduction, a mass of deposited silver (1.0 mg) isdetermined by integrating the current density over time becauseelectrodeposition dominates over galvanic displacement at −1.3 V.

FIG. 6 shows a cauliflower type film composed of densely packednanoparticles, which are produced by electrodeposition of silver on theanodized and etched Al-Cu alloy surface. The coverage of nanoparticlesis about 10¹¹ particles cm⁻² and D_(mean) is 30±7 nm. Comparison of FIG.5 and 6 indicates a dramatic difference in the surface morphologybetween two methods of deposition. Electrodeposition results in theformation of particulate silver films with D_(mean) one order ofmagnitude smaller and the particle coverage two orders of magnitudehigher than those obtained with galvanic displacement type deposition.For spherical particles, S_(g) can be calculated according to Equation(1′), where p is the density of silver (11×10⁶ g/m³). Table 1 summarizesS_(g) obtained with both methods (galvanic displacement andelectrodeposition).S _(g)=6/(ρ×D _(mean) )   (1′)

Due to the small D_(mean) , the fabricated 3-dimensional network ofelectrodeposited silver nanoparticles has a higher S_(g) than thatcomposed of silver particles deposited by galvanic displacement.

Microscopic examination of deposited silver particles was supplementedwith macroscopic electrochemical measurements. In order to confirm thatelectrodeposition results in the formation of silver nanoparticles witha high inner-to-geometric surface area ratio, the electrolyte accessiblesurface area, S_(a), was estimated by EIS. FIGS. 7 and 8 show the Boderepresentation of two EIS data sets collected after galvanicdisplacement type deposition and electrodeposition, respectively.Qualitative analysis of EIS spectra (FIGS. 7 and 8) around 100 Hz and0.5 Hz results in two observations. The magnitude of the total cellimpedance is lower and the phase of the total cell impedance is lessnegative for the electrode with electrodeposited particles of silverthan those for the electrode with particles deposited by galvanicdisplacement. Both observations can be explained by the fact that alarger capacitance makes a smaller contribution to the total cellimpedance in the former case in comparison with the latter case.

For quantitative analysis, EIS spectra are modeled with the equivalentcircuits shown in FIG. 7A and FIG. 9. The equivalent circuit describingparticles deposited by galvanic displacement for 40 min (FIG. 7A) doesnot require considering the 3-dimensional structure of depositedparticles. On the contrary, the equivalent circuit used for modeling ofelectrodeposited particles (FIG. 9) includes the transmission line modeldeveloped for a porous electrode. This model has to be employed becausethe capacitance normalized to the geometric electrode area, C_(area),obtained for the electrode with electrodeposited particles of silverexceeds the specific capacitance of a smooth silver/electrolyteinterface, C_(spec)(20×10⁻⁶ F/cm²), by two orders of magnitude (Table1). Both models were described in details in previous publications.C_(area), and gravimetric capacitances normalized to the mass ofdeposited silver, C_(mass), are shown in Table 1. The electrolyteaccessible surface area of electrodeposited silver particles per gram ofsilver (S_(a), m²/g) can be determined as the ratio of C_(mass) andC_(spec)according to Equation (2′).S_(a)=C _(mass/) C _(spec)=(C _(area)×A)/(m ×C_(spec))   (2′)In Equation (2′), A is the geometric electrode area (1.4 cm²) and m isthe mass of deposited silver. As expected, S_(a) is approximately thesame as S_(g) (Table 1). Thus, the surface area of electrodepositedparticles of silver is completely accessible to the electrolyte. Basedupon these observations, one can conclude that electrodeposition resultsin the deposition of porous silver films composed from electricallyinterconnected nanoparticles of silver. S_(g) of electrodepositedparticles is one order of magnitude higher than that obtained withparticles deposited by galvanic displacement.

It is important to note why the cathodic polarization of the Al-Cuelectrode favors the deposition of silver particles with a highernucleation density than that obtained with galvanic displacement. Thenucleation density is known to exponentially increase with the appliedoverpotential. Therefore, it is not surprising that a high nucleationdensity is obtained when the Al-Cu alloy electrode is polarized to0.6-0.7 V more negative than E_(m) (−0.65 V) established during galvanicdisplacement. The cathodic polarization increases the appliedoverpotential for reduction of silver cations, which translates in ahigh nucleation density.

Particles of silver remain adhesive to the Al-Cu alloy substrate duringelectrodeposition, electrochemical and microscopic examination.Mechanical strength of the fabricated porous structure can be explainedby the fact that the D_(mean) of 30 nm allows for a plenty ofinterconnection points among silver particles per unit volume. Thecorrosion resistance of electrodeposited particles of silver can befurther improved with co-deposition of refractory metals such astungsten.

Electrodeposition of silver on anodized and etched Al-Cu alloysubstrates can be compared with galvanic displacement. In both cases,the combination of anodization and chemical etching results in thecopper enrichment in and underneath the thin layer of barrier aluminumoxide. The enrichment in copper during anodization enables subsequentelectrodeposition of silver in this Example.

In the case of electrodeposition, cathodic polarization allows one tocontrol the nucleation and growth of silver particles and, consequently,properties of deposited silver films. It is worthwhile to point out thatthe technologically important zincation of aluminum substrates isgalvanic displacement type deposition.

In order to increase the technological utility, the procedure toactivate the Al-Cu alloy film for electrodeposition must be compatiblewith standard photolithographic methodology. Previous results obtainedin our laboratory indicated that photoresist did not exhibit acceptableadhesion to the Al-Cu alloy film during porous type anodization. Thisproblem was overcome with the negative pattern transfer technique. Theprocedure for electrodeposition on patterned Al-Cu alloy films issummarized at FIGS. 10A-10F. Silicon wafers with Al-Cu films werepatterned with photoresist, producing two circular patterns withdiameters of 10 and 30 um (FIG. 10A-step 1). The photoresist mask wastransferred to an approximately 100 nm thick layer of barrier aluminumoxide (FIG. 10B-step 2). This layer was formed in the regions notcovered with the photoresist under the following conditions: voltage of90 V, anodization time of 2-3 min, temperature of 3° C. and in 3 % w/vNa₂C₂0₄ (pH 4.5). After removal of photoresist (FIG. 10C-step 3),subsequent 20 min anodization (FIG. 10D-step 4) at voltage of 50 V,temperature of 3° C. and in 3 % w/v H₂C₂0₄ (pH 1.5) resulted in theformation of porous aluminum oxide only in those regions, which were notcovered with the layer of barrier aluminum oxide and were initiallycovered with the photoresist. In order to enable electrodeposition, thewhole layer of porous aluminum oxide and almost all of underlyingbarrier aluminum oxide (1.3 nm left) were chemically etched (FIG.10E-step 5) in a mixture of 0.4 M H₃PO₄ and 0.2 M H₂CrO₄ acids at 50° C.for approximately 90 min as described in the experimental section forblanket Al-Cu films. Electrodeposition of silver (3.0 mM AgNO₃) wasperformed at -1.3 V vs. Hg/HgSO₄ (FIG. 10F-step 6).

FIGS. 11A, 11B demonstrate the state of the alloy surface aftercompletion of all steps in the fabrication procedure (FIGS. 10A-10F).Silver is electrodeposited (FIG. 10F-step 6) only in the circularregions, which underwent porous type anodization (FIG. 10D-step 4). Thespace-selective deposition of silver provides indirect evidence that thecopper enrichment, which occurs during porous typelanodization of Al-Cualloy films, enables the electrodeposition of silver on anodized andchemically etched regions of Al-Cu alloy films. The silver deposits areporous and similar to those shown in the high magnifications micrograph(FIG. 6). FIGS. 11A, 11B confirm that the pretreatment method developedfor activation of Al-Cu alloy films (anodization and chemical etching)is compatible with photolithographic techniques.

To summarize this Example, activation of technologically relevant Al-Cualloy substrates for electrodeposition is achieved by anodizationfollowed by chemical etching of aluminum oxide. Electrodeposition ofsilver on anodized and etched Al-Cu alloy substrates results in thefabrication of a porous film built from electrically interconnectednanoparticles of silver with D_(mean) of 30 nm. The coverage ofelectrodeposited particles of silver is 4×10¹¹ particles cm⁻².Microscopic examination by SEM is supplemented with macroscopicelectrochemical measurements (EIS). EIS is shown to be a useful in-situmethod to monitor the surface area of deposited particulate films. Thefrequency response of the porous network of electrodeposited silvernanoparticles is evaluated using the transmission line model. Thecapacitance normalized to the geometric electrode area is 2.9±0.1 mF/cm²and the capacitance normalized to the mass of deposited silver is3.9±0.1 mF/g. The electrolyte accessible area of electrodeposited silvernanoparticles is 20 m²/g. The method developed for electrodeposition onanodized and etched Al-Cu alloy films is compatible withphotolithographic techniques. Electrodeposition on patterned Al-Cu alloyfilms is accomplished by transferring the photoresist mask to a layer ofbarrier aluminum oxide. This layer acts as a mask for porous typeanodization. Following anodization and chemical etching,electrodeposition of silver takes place only on anodized and etchedareas. Table 1. The mean particle diameter, D_(mean) ; specific surfacearea, S_(g); mass of deposited silver, m; capacitance normalized to thegeometric electrode area; C_(area), gravimetric capacitances; C_(mass),and electrolyte accessible surface area; S_(a), for silver particlesdeposited by either galvanic displacement type deposition orelectrodeposition. D_(mean) (nm) S_(g) (m²/g) m C_(area) C_(mass) S_(a)Process (SEM) (SEM) (mg) (mF/cm²) (F/g) (m²/g) Galvanic 180 ± 110 3.0 ±1.8 N/A 0.058 ± 0.002 N/A N/A displacement ElectroDeposition 30 ± 7  18± 4  1.0 2.9 ± 0.1 3.9 ± 0.2 19 ± 1Although the invention has been described in connection with certainembodiments thereof, those skilled in the art will appreciate that theinvention is not limited to these illustrative embodiments and thatchanges and modifications can be made thereto within the scope of theinvention as set forth in the following claims.

1. Method of depositing a metallic material on a substrate, comprisingthe steps of providing a substrate comprising an alloy of aluminum andan alloying element, oxidizing a surface of the substrate to formaluminum oxide thereon, etching the oxidized surface to leave a partialthickness of a barrier aluminum oxide of said aluminum oxide on thesurface, and depositing by galvanic displacement type deposition,electroless deposition or electrodeposition discrete metallicnanoparticles on the barrier oxide having a particle density of about10⁴ to about 10¹² particle/cm².
 2. The method of claim 1 wherein saidetching is conducted to leave a barrier oxide portion having a partialthickness to increase nucleation of the discrete nanoparticles thereon.3. The method of claim 1 wherein the substrate comprises a film or layerof the alloy.
 4. The method of claim 1 wherein the alloying elementincludes one or more of Au, Cu, Cr, Mn, Mo, Ni, Si, Ta, Ti, or Zn. 5.The method of claim 1 wherein the surface is oxidized by anodizing,polishing, alkaline etching, acid pickling, electropolishing, or heatingin an oxygen bearing atmosphere.
 6. The method of claim 1 wherein thesurface is acid etched by contact with a mixture of phosphoric acid andan inhibitor for aluminum dissolution.
 7. The method of claim 6 whereinthe inhibitor comprises chromic acid.
 8. The method of claim 1 whereinthe metallic material comprises one of Au, Ag, Pd, Cu, Ni, Pb, Cr, Fe,W, Mo, or Co.
 9. The method of claim 1 wherein the substrate is disposedon a silicon wafer.
 10. A substrate comprising an alloy of aluminum andan alloying element, said substrate having a barrier oxide on asubstrate surface and a porous, three dimensional structure ofelectrically interconnected, metallic nanoparticles deposited byelectroless deposition or electrodeposition on the barrier oxide. 11.The substrate of claim 10 wherein the structure includes randomlypacked, generally spherical metallic nanoparticles having a distributionof particle sizes.
 12. The substrate of claim 10 wherein thenanoparticles have a particle diameter in the range of about 20 nm toabout 1000 nm.
 13. The substrate of claim 10 wherein the nanoparticleshave an interparticle spacing sufficient to provide electrolyte accessamong the nanoparticles, providing a high surface area material.
 14. Thesubstrate of claim 10 wherein the structure has a ratio betweenelectrolyte accessible area and geometric surface area of about 100 andabove.
 15. Electrode comprising a substrate comprising an alloy ofaluminum and an alloying element, said substrate having a barrier oxideon a substrate surface and a porous, electrolyte accessible, threedimensional structure of electrically interconnected, generallyspherical, randomly packed metallic nanoparticles deposited by galvanicdisplacement type deposition, electroless deposition orelectrodeposition on the barrier aluminum oxide.
 16. The electrode ofclaim 15 wherein the nanoparticles have a particle diameter in the rangeof about 20 nm to about 1000 nm.
 17. The electrode of claim 15 whereinthe structure has a ratio between electrolyte accessible area andgeometric surface area of about 100 and above.
 18. The electrode ofclaim 15 wherein the barrier oxide film has been chemically etched. 19.The electrode of claim 15 wherein the substrate is disposed on a siliconwafer.
 20. Capacitor having an electrode in accordance with claim 15.21. Method of depositing a metallic material on a substrate, comprisingthe steps of providing a substrate comprising an alloy of aluminum andan alloying element, oxidizing a surface of the substrate to formaluminum oxide thereon, etching the oxidized surface to leave a partialthickness of a barrier aluminum oxide of said aluminum oxide on thesurface, and depositing by galvanic displacement type deposition,electroless deposition or electrodeposition a continuous metallic filmon the barrier oxide.
 22. The method of claim 21 wherein said etching isconducted to leave a barrier oxide portion having a partial thickness todeposit a continuous metallic film.
 23. The method of claim 21 whereinthe substrate comprises a film or layer of the alloy.
 24. The method ofclaim 21 wherein the alloying element includes one or more of Au, Cu,Cr, Mn, Mo, Ni, Si, Ta, Ti, or Zn.
 25. The method of claim 21 whereinthe surface is oxidized by anodizing, polishing, alkaline etching, acidpickling, electropolishing, or heating in an oxygen bearing atmosphere.26. The method of claim 21 wherein the surface is acid etched by contactwith a mixture of phosphoric acid and an inhibitor for aluminumdissolution.
 27. The method of claim 26 wherein the inhibitor compriseschromic acid.
 28. The method of claim 21 wherein the metallic materialcomprises one of Au, Ag, Pd, Cu, Ni, Pb, Cr, Fe, W, Mo, or Co.
 29. Themethod of claim 21 wherein the substrate is disposed on a silicon wafer.